Hydraulic motion compensating apparatus

ABSTRACT

A compensating apparatus is suspended from a divided crown block mounted on a derrick of a floating drilling barge, the compensating apparatus including a single cylinder extending vertically between and carrying divided travelling block sheaves on opposite sides thereof, with drawwork lines passing over the divided crown block sheaves and divided travelling block sheaves, and with the upper portion of the cylinder extending between such lines and adapted to move upwardly between the crown block sheaves without interference. A piston structure, forming part of the compensating apparatus, is slidable in the cylinder, being connected to a hook, or the like, from which a running string, such as a drilling string, is suspended extends into an underlying well bore, liquid under pressure in the cylinder acting on the piston structure to support part or all of the weight of the running string. The piston structure and cylinder can be locked together mechanically when in a fully contracted condition to render the compensating apparatus ineffective.

The present invention relates to apparatus for controlling the stress ina running string, and more particularly to apparatus used on or inconnection with a floating vessel or barge for maintaining the strain ina running string, such as a pipe string, substantially constant whilebeing used in the performance of diverse functions in a subaqueous wellbore, such as drilling and completion operations therein, despitevertical movement of the vessel while such operations are beingperformed.

It has been proposed to provide a compensating apparatus between thetravelling block and hook of a well bore ring mounted on a vesselfloating in a body of water and used in drilling a subaqueous well bore,or performing other operations therein, to maintain the stress in arunning string, such as a string of drill pipe, used in drilling thewell bore underlying the body of water, at a substantially constantvalue. Such compensating apparatus includes a cylinder supported fromthe travelling block and a piston slidable therein connected to thehook, a desired liquid pressure being maintained in the cylinder forexerting an upward supporting force on the piston, which supportingforce remains substantially constant despite relative telescopicmovement between the cylinder and piston portions of the compensatingapparatus, resulting from heaving of the drilling vessel under tide,wind and wave conditions.

In the application of James W. E. Hanes and Edward Larralde, Ser. No.69,759, filed Sept. 4, 1970, for "Motion Compensating Apparatus," nowPat. No. 3,714,995 it has been proposed to avoid reduction in the extentof vertical movement available in a derrick mounted on the floatingvessel, as a result of interposing a compensating apparatus between thetravelling block and hook. The length added between the travelling blockand hook materially lessens the available maximum travel of thedrawbacks or hoisting apparatus, requiring the raising and lowering ofshorter stands of drill pipe in the well bore, which thereby increasesthe time required in making a round trip of the pipe in the well bore.In U.S. Pat. No. 3,714,995, significant height saving advantages havebeen realized when used with standard crown blocks from which thetravelling block is suspended by the drawworks lines. Such heightadvantages result from employing a pair of cylinders and pistons whichstraddle or overlap the travelling block, such pair of cylinders andpistons of necessity being eccentric with respect to the center line oraxis of the travelling block and hook suspended therebelow, whichintroduces a bending or binding tendency in the compensating apparatus.

By virtue of the present invention, a compensating apparatus is providedwhich is so related to the travelling block and hook as to provide foran increase in the maximum stroke of the compensating apparatus withoutadversely affecting the required height of the derrick.

A further objective of the present invention is to provide a dividedcrown block, with a single compensating cylinder and associated pistonsuspended therefrom which will be concentrically disposed with respectto the hook therebelow, in order that the mass of the apparatus and theforces acting thereon are applied in a concentric manner, therebyeliminating any tendency for bending, binding or seizing to occur inparts of the apparatus, and also minimizing wear of the seals andbearings embodied in the apparatus.

Yet a further object of the invention is to employ the cylinder of thecompensating apparatus as a portion of the travelling block itself, asby mounting the sheaves of the travelling block on opposite sides of thecylinder, this arrangement being employed in connection with a dividedcrown block, the divided or split crown block having a gap therebetweeninto which the cylinder can move, thereby resulting in increase in thestroke or travel of the apparatus.

A further object of the invention is to provide a lock device betweenthe cylinder and piston portions of the compensating apparatus which isconcentrically disposed with respect to the cylinder and piston portionsof the apparatus, enabling the compensating apparatus to be renderedinoperative during "round tripping" of the drill pipe, or the like,suspended from the apparatus, the lock being operative upon fulltelescopic or contracted movement of the piston structure within thecylinder. The locking together of the travelling block and hook, andtheir release from one another, can be effected from a remote pointunder the control of the operator.

This invention possesses many other advantages, and has other objectswhich may be made more clearly apparent from a consideration of a formin which it may be embodied. This form is shown in the drawingsaccompanying and forming part of the present specification. It will nowbe described in detail, for the purpose of illustrating the generalprinciples of the invention; but it is to be understood that suchdetailed description is not to be taken in a limiting sense.

REFERRING TO THE DRAWINGS

FIG. 1 is a diagrammatic view of an apparatus embodying the invention,illustrated in connection with a rig mounted on a floating vessel fordrilling a well bore underlying a body of water;

FIG. 2 is a diagrammatic, enlarged view, parts being shown in section,of the compensating apparatus illustrated in FIG. 1;

FIGS. 3a, 3b and 3c together constitute an enlarged quarter sectionalview, parts being shown in elevation, of the apparatus disclosed in FIG.2, FIGS. 3b and 3c being lower continuations of FIGS. 3a and 3b,respectively; and

FIGS. 4a and 4b together constitute an enlarged longitudinal section ofthe apparatus disclosed in FIGS. 3a, 3b and 3c, parts being illustratedin elevation, disclosing the compensating apparatus fully contracted andlocked in an inoperative position, FIG. 4b, being a lower continuationof FIG. 4a.

An apparatus is illustrated in the drawings in connection with thedrilling of a vertical well bore W from a subaqueous floor F above whicha drilling barge B, or other floating vessel, is located, the bargebeing suitably anchored against lateral displacement for the purpose ofholding a drilling string S in centered relation with respect to thewell bore. A drill bit A is secured to the lower end of the drillstring, such as a string of drill pipe .Iadd.or drill .Iaddend.casing,the upper or kelly portion K of the drill string passing through theusual rotary table T rotated by a suitable drive mechanism (not shown).The upper end of the kelly is secured to a swivel C, which is, in turn,suspended from a hook H connected to the lower portion of a motioncompensating apparatus D operatively connected to a travelling blockstructure E suspended from a crown block structure M, mounted on theupper portion of the derrick N, through the use of the usual lines Lconnected to a drawworks P mounted on the drilling barge, the drawworkspermitting the drill string S to lower as the well bore W is cut by thebit A.

A mudline R is connected to the swivel C for the purpose of pumpingdrilling mud, or the like, down the drill string S for discharge fromthe bit A, for the purpose of removing the cuttings produced by thelatter. While the drill bit A is rotated by the drill string S,appropriate drilling weight is imposed thereon. Usually, the drillingweight is provided by a suitable length of drill collars disposed in thelower portion of the drill string immediately above the drill bit, thedrill string above the drill collars being maintained in tension by thedrawworks P, lines L and compensating apparatus D.

The compensating apparatus D permits the floating vessel B and themechanism carried thereby to shift vertically relative to the well boreW, without appreciably modifying the stress in the drill string S, and,therefore, the drilling weight imposed on the drill bit A. Throughmaintenance of liquid pressure in the compensating apparatus, thetension or strain in the drill string S is maintained substantiallyconstant at a selected value, despite the heaving of the floating vesselin the body of water. A system for maintaining pressure of the liquid inthe cylinder 10 of the compensating apparatus at a selected constantvalue includes an accumulator arrangement 11, disclosed diagrammaticallyin FIG. 1, in which the pressure of a gas 12 in the accumulator istransferred through a piston 13 to a liquid medium 14 in theaccumulator, and through the lines 15, 15a to the cylinder of thecompensating apparatus. One such system is illustrated in theapplication of Edward Larralde, Ser. No. 222,919, filed Feb. 2, 1972,for "Variable Rate Hydraulic-Pneumatic Weight Control and CompensatingApparatus."

As specifically shown, the compensating apparatus includes the cylinderstructure 10, embodying an elongate outer cylinder sleeve 16 threadedlyconnected at its upper end to an upper cylinder head 17, and at itslower end to a lower cylinder head 18. Disposed within the outercylinder sleeve 16 is a coaxial inner cylinder sleeve 19 threadedlysecured to the upper head 17 and terminating in a lower head 20. Apiston structure 21 is shiftable longitudinally along both inner andouter cylinder sleeves 19, 16, the structure including an upper annularpiston 22 slidably sealing against the inner wall 23 of the outer sleeve16 and the outer wall 24 of the inner sleeve, leakage between the pistonand sleeves being prevented by suitable inner and outer seal rings 25,26 mounted on the piston and sealingly engaged with the outer and innerwalls 24, 23. This piston is threadedly or otherwise suitably secured toa tubular piston rod 27, the outer periphery 28 of which is spacedlaterally from the inner cylinder wall 23, to define an annular cylinderspace 29 therebetween, the piston rod also being spaced laterally fromthe inner sleeve 19 to provide an annular space 30 therebetween. Theinner head 20 is in slidable relation to the inner wall 31 of thetubular piston rod, leakage of fluid therebetween being prevented bysuitable seal rings 32 on the inner head slidably and sealingly engagingagainst the inner wall of the tubular piston rod. The lower cylinderhead 18 carries a suitable rod seal 33 sealingly engaging against theperiphery 28 of the piston rod 27.

The piston rod 27 extends through the lower cylinder head 18 and has itslower portion threadedly secured to a lower lock member 35 suitablysecured, as by means of a plurality of screws 36, to an adapter 37having an upper inner flange 37a resting upon an outer flange 35a of thelock member 35 and forming part of a gimbal 38 secured to the lower hookH, which depends therefrom and which is coaxially arranged with respectto the cylinder sleeves 16, 19 and the piston structure 21. The gimbalincludes trunnions 39 extending from opposite sides of a cross-head 40,and which are pivotally mounted in the adapter 37, the cross-head havinga transverse pin 41 connected to the upper portion 42 of the hook andlying in the same plane as the trunnions 39, being disposed 90 degreeswith respect thereto. It is apparent that the hook H can pivot about theaxis of the transverse pin 41 and also about the axis of the trunnions39, which provides for a universal movement of the hook with respect tothe piston rod 27.

The travelling block structure E is actually integrated into thecylinder 10. Thus, the cylinder sleeve 16 has trunnions or bearingsupports 42 suitably secured, as by use of welding material 43, onopposite sides of the cylinder substantially below the upper cylinderhead 17, there being a plurality of sheaves 44 rotatably mounted on eachof said trunnions through suitable intervening roller or other bearings(not shown), the bearings and a housing 45 enclosing the lower portionof the sheaves being retained on the trunnions 42 by a suitable nut 46threaded on the outer portion of each trunnion.

It is apparent that the travelling block E is split or divided, beingarranged on opposite sides of the cylinder 10, which permits thedrawworks lines L to pass successfully over such sheaves 44 andcorresponding sheaves 50 forming a part of the crown block M, the crownblock being split or divided and providing a gap 51 between its twoportions. It is apparent that the crown block is suitably supported onthe upper end of the derrick N.

The travelling block E and the compensator apparatus D secured theretocan be elevated by the drawworks P to a maximum position in which thesplit travelling block sheaves 44 are disposed closely adjacent to thecrown block sheaves 50, the cylinder 10 entering the gap or space 51between the crown block sheaves. Accordingly, a maximum range of travelis achievable, offsetting the length added to the apparatus by thecompensator portion D of the apparatus disposed below the sheaves 44 andintervening between the sheaves 44 and the hook H.

The liquid under pressure from the accumulator 11 can pass into and outof the annular cylinder space 29 through an inlet and outlet port 60 inthe lower cylinder head to which one of the liquid lines 15 is suitablyconnected. The liquid under pressure in such annular cylinder space actsin an upward direction over the annular piston 22 across the annulararea of the cylinder space 29 to exert an upward force on the piston,the tubular piston rod 27 and the hook H and other apparatus suspendedtherefrom. Similarly, the liquid under pressure can pass through theline 15a connected to an inlet and outlet port 61 in the upper cylinderhead 17, and through a central passage 62 in the inner cylinder sleeve19 which communicates through a lower port 63, immediately above theinner head 20, with the annular space 30 between the outer wall 24 ofthe inner cylinder sleeve and the tubular piston rod 27, this liquidunder pressure also acting in an upward direction over the inner portionof the annular piston 22 and assisting in the supporting of the pistonrod 27 and the mechanism suspended therefrom.

As the drilling barge or vessel heaves in the water, the liquid underpressure will pass into and out of the annular cylinder spaces 29, 30,permitting longitudinal travel of the outer and inner cylinders 16, 19along the piston rod 27 without substantially affecting the pressure ofthe liquid in the cylinder spaces 29, 30, which continues to exert asubstantially constant upward force on the piston 22 and the piston roddepending therefrom, the liquid passing into and from the accumulator11, with the pressure of the gas 12 maintaining the substantiallyconstant unit pressure in the liquid 14. Thus, a constant stress ortension is maintained on the drilling string S, without varying itsrelative position within the well bore W as the drilling barge B andcylinder 10 move vertically under the action of wind, wave and tide.

At times, it is desirable for the motion compensator apparatus D to berendered inoperative, as, for example, during withdrawal of the drillpipe S, or its relowering from the drilling barge into the well bore.With the apparatus illustrated, the cylinder 10 and piston structure 21are mechanically locked together when the piston 22 occupies itsuppermost position within the cylinder 10; in effect, securing the hookH in a predetermined position with respect to the travelling block E sothat they operate as a unit during the raising and lowering of thetravelling block. As disclosed, the lower lock member 35 secured to thepiston rod 27 has an external lock flange 70 thereon provided with adownwardly facing lock surface 71. The lower cylinder head 18 has anupper lock member 72 threadedly secured thereto and depending therefrom,and through which the piston rod 27 is slidable, which also has an upperexternal lock flange 73 thereon provided with an upwardly tapering locksurface 74, which are oppositely disposed with respect to the lower lockflange 70 and its lock surface 71. A collet type of lock device 75 iscarried by the upper lock member 72, this lock device including an upperhead 76 suitably secured to the upper lock member 72 by means of aplurality of screws 77a. Integral with and depending from the head 76are a plurality of circumferentially arranged spring-like arms 77terminating in lock dogs 78, each dog having an inner recess 79 in whichthe upper and lower lock flanges 73, 70 can be received when they aredisposed adjacent to one another. The dogs 78 have upper lock surfaces80 and lower lock surfaces 81 spaced from one another and defining theupper and lower ends of the recesses 79, and which conform to the upperand lower lock surfaces 74, 71 on the flanges 73, 70.

Normally, the spring arms 77 shift the lock dogs 78 laterally outwardlyinto a position permitting upward movement of the lower lock flange 70through the lower ends of the lock dogs, and into a positionsubstantially contacting the upper lock flange 73, as illustrated inFIG. 4b. With the lock flanges in such abutting or adjacent position,the lock dogs 78 can be shifted inwardly to a position in which theupper and lower lock dog surfaces 80, 81 engage the companion locksurfaces 74, 71 on the lock flanges 73, 70, thereby securing the piston21 and cylinder 10 to one another, preventing their relativelongitudinal movement,

As specifically disclosed, the actuation of the lock device 75 may beeffected from a remote point. Thus, an annular piston 85 is providedwithin a lock cylinder 86, the upper portion of which is threadedlysecured to the lower cylinder head 18, which actually constitutes anupper head 87 of the lock cylinder. The annular piston is disposed inthe space between the upper lock member 72 and the cylinder 86, anactuator sleeve 88 integral with and depending from this piston 85 beingprovided which extends downwardly within a lower cylinder portion 89 ofsubstantially lesser internal diameter than the upper portion of thecylinder 86, and in which the sleeve 88 is slidable. The spring arms 77and lock dogs 78 are disposed within the lower cylinder portion 89. Thelower portion of the sleeve is constituted as an internal cam 90 havingits inner surface inclined in a downward and outward direction andadapted to engage companion cam surfaces 91 on the lock dogs, for thepurpose of shifting the dogs 78 laterally inwardly upon downwardmovement of the piston 85 and its sleeve 88 within the cylinder 86. Suchdownward movement occurs as a result of feeding hydraulic fluid througha line 94 connected to the upper portion of the cylinder 86 and adaptedto direct liquid under pressure into the cylinder above the piston 85,for the purpose of forcing the piston 85 and its cam 90 downwardly whenthe lower lock member 35 abuts the upper lock member 72, causing the camto shift the lock dogs 78 inwardly to the locking position illustratedin FIG. 4b. The angle of taper of the cam surface 90 and the companionsurfaces 91 on the dogs is relatively small, actually constituting aself-locking angle, such that the dogs 78 are retained in a positionlocking the piston structure 21 and cylinder structure 10 againstlongitudinal movement with respect to each other in the absence of fluidpressure in the cylinder 86 above the lock piston 85.

In the event the lock is to be released, liquid under pressure is forcedthrough a suitable line 95 communicating with the cylinder 86 below thepiston when it is in its lower position, this fluid acting in an upwarddirection on the piston, to elevate the piston 85 and its associatedsleeve 88 within the cylinder 86 and relative to the lock dogs 78,placing the piston and sleeve in the position illustrated in FIG. 3b.When the cam surface 90 is out of .[.arrangement.]. .Iadd.engagement.Iaddend.with the lock dogs 78, the spring arms 77 will inherentlyexpand to their outer position, permitting the piston rod 27 to movefreely within the cylinder 10. If this were not to occur, a relativedownward force exerted on the piston rod 27, as by the weight suspendedtherefrom, will cause its tapered lock surface 71 to force the lock dogs78 to their lateral outward or unlocked position illustrated in FIG. 3b.

By virtue of the apparatus described, a compensating apparatus has beenprovided that permits a desired maximum movement of the travelling blockwithout requiring an undue derrick height. The divided crown block M andthe straddling relationship between the cylinder 10 and the sheaves 44permit the apparatus to be elevated within the derrick to such an extentthat the full required elevation of the drilling string within thederrick can occur, in essentially the same manner as if a compensatingapparatus were not present. The dividing of the crown block permits thecylinder 10 to pass between its two portions without interference,enabling a greater elevation of the travelling block to occur, ascompared to the provision of an unsplit or undivided crown block. Evenin the absence of a split crown block, the required travel of thetravelling block E can still occur without interference by thecompensator apparatus D, because of the straddled or overlappingrelation between the split travelling block and the compensator cylinder10. The provision of a single compensator cylinder and a single pistonoperating therein, related in a concentric manner to the apparatussuspended therefrom, results in forces being applied to the severalparts in a concentric manner, thus eliminating bending, binding orseizing tendencies on the parts, as well as minimizing loads on sealsand bearing surfaces. The concentric locking device enables thecompensator portion of the apparatus to be rendered inoperative,permitting joint movement of the travelling block E and the hook H tothe same extent as if the compensating apparatus D were not present.With the apparatus in a locked condition, there is no necessity forretaining liquid under pressure in the cylinder 10. The concentriclocking device is simpler than the type of locking device illustrated inU.S. Pat. No. 3,714,995, the ease of effecting locking and unlockingbeing greatly increased. Such selective locking or unlocking of thetravelling block and hook can be effected from a remote point under thecontrol of the operator.

We claim:
 1. An apparatus for maintaining a predetermined stress in arunning string disposed in a well bore and which is supported by a rig,including a derrick and a suspension mechanism supported from the upperportion of the derrick: comprising a travelling block structure adaptedto form part of the suspension mechanism; a hook structure adapted tosupport the running string therebelow; a motion compensating apparatusdisposed between and connected to said block structure and hookstructure; said motion compensating apparatus comprising cylinder meanscoaxial of said hook structure; piston means slidable in said cylindermeans and connected to said hook structure; said travelling blockstructure being split and fixedly mounted to said cylinder means onopposite sides of said cylinder means substantially below the upper endof said cylinder means and with said cylinder means disposed betweensaid split block structure; and means for maintaining a fluid mediumunder pressure in said cylinder means for exerting an upward force onsaid piston means as said piston means and cylinder means movelongitudinally relative to one another in both longitudinal directions.2. An apparatus as defined in claim 1; said block structure comprisingone or more sheaves rotatably mounted on said cylinder means at one sidethereof and one or more sheaves rotatably mounted on said cylinder meansat the opposite side thereof.
 3. An apparatus as defined in claim 1;said block structure comprising one or more sheaves rotatably mounted onsaid cylinder means at one side thereof and one or more sheavesrotatably mounted on said cylinder means at the opposite side thereof;said cylinder means projecting upwardly beyond said sheaves.
 4. Anapparatus as defined in claim 1; and means for releasably locking saidpiston means and cylinder means to each other when said piston means issubstantially fully telescoped within said cylinder means to preventrelative longitudinal movement between said piston means and cylindermeans.
 5. An apparatus as defined in claim 1; collet means forreleasably locking said piston means and cylinder means to each otherwhen said piston means is substantially fully telescoped within saidcylinder means to prevent relative longitudinal movement between saidpiston means and cylinder means; and means movable longitudinally alongone of said cylinder means and piston means for selectively shiftingsaid collet means to locking or unlocking position.
 6. An apparatus asdefined in claim 1; collet means for releasably locking said pistonmeans and cylinder means to each other when said piston means issubstantially fully telescoped within said cylinder means to preventrelative longitudinal movement between said piston means and cylindermeans; and hydraulically operable means movable longitudinally alongsaid cylinder means for selectively shifting said collet means tolocking or unlocking position.
 7. An apparatus as defined in claim 1;said cylinder means comprising an outer member and an inner membertherewithin and secured to said outer member, said inner and outermembers being laterally spaced from each other to provide an annularspace therebetween in which said piston means is movable; said pistonmeans comprising a piston in said annular space slidably sealing againstthe inner wall of said outer member and the outer wall of said innermember; said piston means further comprising a piston rod secured tosaid piston and laterally spaced from said inner wall of said outermember and in slidable sealed relation to said outer and inner members;and means for directing said fluid medium under pressure into the spacebetween said piston rod and inner wall of said outer member for actionupon said piston; said piston rod being laterally spaced from said innermember to provide a second annular space therebetween; and means fordirecting said fluid medium under pressure into said second annularspace for action upon said piston.
 8. An apparatus as defined in claim1; said cylinder means comprising an outer member and an inner membertherewithin and secured to said outer member, said inner and outermembers being laterally spaced from each other to provide an annularspace therebetween in which said piston means is movable; said pistonmeans comprising a piston in said annular space slidably sealing againstthe inner wall of said outer member and the outer wall of said innermember; said piston means further comprising a piston rod secured tosaid piston and laterally spaced from said inner wall of said outermember and in slidable sealed relation to said outer and inner members;and means for directing said fluid medium under pressure into the spacebetween said piston rod and inner wall of said outer member for actionupon said piston; and means for releasably locking said piston rod andouter member to each other when said piston and piston rod aresubstantially fully telescoped within said cylinder means to preventrelative longitudinal movement between said piston means and cylindermeans.
 9. An apparatus as defined in claim 1; said cylinder meanscomprising an outer member and an inner member therewithin and securedto said outer member, said inner and outer members being laterallyspaced from each other to provide an annular space therebetween in whichsaid piston means is movable; said piston means comprising a piston insaid annular space slidably sealing against the inner wall of said outermember and the outer wall of said inner member; said piston meansfurther comprising a piston rod secured to said piston and laterallyspaced from said inner wall of said outer member and in slidable sealedrelation to said outer and inner members; and means for directing saidfluid medium under pressure into the space between said piston rod andinner wall of said outer member for action upon said piston; colletmeans for releasably locking said piston rod and outer member to eachother when said piston means is substantially fully telescoped withinsaid cylinder means to prevent relative longitudinal movement betweensaid piston means and cylinder means; and means movable longitudinallyalong said outer member for selectively shifting said collet means tolocking or unlocking position.
 10. An apparatus for maintaining apredetermined stress in a running string disposed in a well bore andwhich is supported by a rig, including a derrick: comprising asuspension mechanism including a split crown block structure adapted tobe supported from the upper portion of the derrick; a travelling blockstructure below said crown block structure and suspended therefrom bylines passing around both structures; a hook structure adapted tosupport the running string therebelow; a motion compensating apparatusdisposed between and connected to said travelling block structure andhook structure; said motion compensating apparatus comprising cylindermeans coaxial of said hook structure; piston means slidable in saidcylinder means and connected to said hook structure; said travellingblock structure being split and fixedly mounted to said cylinder meanson opposite sides of said cylinder means substantially below the upperend of said cylinder means and with said cylinder means disposed betweensaid split block structure; and means for maintaining a fluid mediumunder pressure in said cylinder means for exerting an upward force onsaid piston means as said piston means and cylinder means movelongitudinally relative to one another in both longitudinal directions.11. An apparatus as defined in claim 10, said crown block structurecomprising sets of sheaves laterally spaced from each other; saidtravelling block structure comprising one or more sheaves rotatablymounted on said cylinder means at one side thereof and one or moresheaves rotatably mounted on said cylinder means at the opposite sidethereof.
 12. An apparatus as defined in claim 10; said crown blockstructure comprising sets of sheaves laterally spaced from each other;said travelling block structure comprising one or more sheaves rotatablymounted on said cylinder means at one side thereof and one or moresheaves rotatably mounted on said cylinder means at the opposite sidethereof; said cylinder means projecting upwardly beyond, said travellingblock sheaves and adapted to extend between said sets of sheaves of saidcrown block structure.
 13. An apparatus as defined in claim 10; saidcylinder means comprising an outer member and an inner membertherewithin and secured to said outer member, said inner and outermembers being laterally spaced from each other to provide an annularspace therebetween in which said piston means is movable; said pistonmeans comprising a piston in said annular space slidably sealing againstthe inner wall of said outer member and the outer wall of said innermember; said piston means further comprising a piston rod secured tosaid piston and laterally spaced from said inner wall of said outermember and in slidable sealing relation to said outer and inner members;and means for directing said fluid medium under pressure into the spacebetween said piston rod and inner wall of said outer member for actionupon said piston.
 14. An apparatus as defined in claim 10; said cylindermeans comprising an outer member and an inner member therewithin andsecured to said outer member, said inner and outer members beinglaterally spaced from each other to provide an annular spacetherebetween in which said piston means is movable; said piston meanscomprising a piston in said annular space slidably sealing against theinner wall of said outer member and the outer wall of said inner member;said piston means further comprising a piston rod secured to said pistonand laterally spaced from said inner wall of said outer member and inslidable sealing relation to said outer and inner members; and means fordirecting said fluid medium under pressure into the space between saidpiston rod and inner wall of said outer member for action upon saidpiston; and means for releasably locking said piston rod and outermember to each other when said piston means is substantially fullytelescoped within said cylinder means to prevent relative longitudinalmovement between said piston means and cylinder means.
 15. An apparatusas defined in claim 10, said cylinder means comprising an outer memberand an inner member therewithin and secured to said outer member, saidinner and outer members being laterally spaced from each other toprovide an annular space therebetween in which said piston means ismovable; said piston means comprising a piston in said annular spaceslidably sealing against the inner wall of said outer member and theouter wall of said inner member; said piston means further comprising apiston rod secured to said piston and laterally spaced from said innerwall of said outer member and in slidable sealing relation to said outerand inner members; and means for directing said fluid medium underpressure into the space between said piston rod and inner wall of saidouter member for action upon said piston; collet means for releasablylocking said piston rod and outer member to each other when said pistonmeans is substantially fully telescoped within said cylinder means toprevent relative longitudinal movement between said piston means andcylinder means; and means movable longitudinally along said outer memberfor selectively shifting said collet means to locking or unlockingposition. .Iadd.
 16. An apparatus useful for maintaining a predeterminedstress in a running string positioned in well bore and supported by arig including suspension mechanism for which the running string issuspended, said apparatus including a travelling block structure adaptedto form part of the suspension mechanism, a support structure adapted tosupport the running string therebelow, a motion compensating apparatusdisposed between and connected to said travelling block structure andsupport structure, said motion compensating apparatus comprisingcylinder means coaxial of said support structure, piston means slidablein said cylinder means and connected to said support structure, saidtravelling block structure being split and fixedly mounted to saidcylinder means on opposite sides of said cylinder means substantiallybelow the upper end of said cylinder means and with said cylinder meansdisposed between said split travelling block structure, and means onsaid cylinder means for connection to a source of fluid pressure formaintaining a fluid medium under pressure in said cylinder means forexerting an upward force on said piston means as said piston means andcylinder means move longitudinally relative to one another in bothlongitudinal directions. .Iaddend..Iadd.
 17. An apparatus as defined inclaim 16; said travelling block structure comprising one or more sheavesrotatably mounted on said cylinder means at one side thereof and one ormore sheaves rotatably mounted on said cylinder means at the oppositeside thereof. .Iaddend. .Iadd.
 18. An apparatus as defined in claim 16;said travelling block structure comprising one or more sheaves rotatablymounted on said cylinder means at one side thereof and one or moresheaves rotatably mounted on said cylinder means at the opposite sidethereof, said cylinder means projecting upwardly beyond saidsheaves..Iaddend. .Iadd.
 19. An apparatus as defined in claim 16; andmeans for releasably locking said piston means and cylinder means toeach other when said piston means is substantially fully telescopedwithin said cylinder means to prevent relative longitudinal movementbetween said piston means and cylinder means..Iaddend. .Iadd.
 20. Anapparatus as defined in claim 16; said cylinder means comprising anouter member and an inner member therewithin and secured to said outermember, said inner and outer members being laterally spaced from eachother to provide an annular space therebetween in which said pistonmeans is movable, said piston means comprising a piston in said annularspace slidably sealing against the inner wall of said outer member andthe outer wall of said inner member, said piston means furthercomprising a piston rod secured to said piston and laterally spaced fromsaid inner wall of said outer member and in slidable sealed relation tosaid outer and inner members, means for directing a fluid medium underpressure into the space between said piston rod and inner wall of saidouter member for acting upon said piston, said piston rod beinglaterally spaced from said inner member to provide a second annularspace therebetween, and means for directing said fluid medium underpressure into said second annular space for action upon said piston..Iaddend. .Iadd.
 21. An apparatus useful for maintaining a predeterminedstress in a running string disposed in a well bore and which issupported by a rig which includes a derrick; said apparatus including asuspension mechanism including a split crown block structure adapted tobe supported from the upper portion of the derrick, a travelling blockstructure below said crown block structure and suspended therefrom bylines passing around both structures, a support structure adapted tosupport the running string therebelow, a motion compensating apparatusdisposed between and connected to said travelling block structure andsupport structure, said motion compensating apparatus comprisingcylinder means coaxial of said support structure, piston means slidablein said cylinder means and connected to said support structure, saidtravelling block structure being split and fixedly mounted to saidcylinder means on opposite sides of said cylinder means substantiallybelow the upper end of said cylinder means and with said cylinder meansdisposed between said split block structure, and means for directing afluid medium under pressure into said cylinder means for exerting anupward force on said piston means as said piston means and cylindermeans move longitudinally relative to one another in both longitudinaldirections..Iaddend. .Iadd.
 22. An apparatus as defined in claim 21;said crown block structure comprising sets of sheaves laterally spacedfrom each other, said travelling block structure comprising one or moesheaves rotatably mounted on said cylinder means at one side thereof andone or more sheaves rotatably mounted on said cylinder means at theopposite side thereof..Iaddend. .Iadd.
 23. An apparatus as defined inclaim 21; said crown block structure comprising sets of sheaveslaterally spaced from each other, said travelling block structurecomprising one or more sheaves rotatably mounted on said cylinder meansat one side thereof and one or more sheaves rotatably mounted on saidcylinder means at the opposite side thereof, said cylinder meansprojecting upwardly beyond said travelling block sheaves and adapted toextend between said sets of sheaves of said crown blockstructure..Iaddend. .Iadd.
 24. An apparatus as defined in claim 21; saidcylinder means comprising an outer member and an inner membertherewithin and secured to said outer member, said inner and outermembers being laterally spaced from each other to provide an annularspace therebetween in which said piston means is movable, said pistonmeans comprising a piston in said annular space slidably sealing againstthe inner wall of said outer member and the outer wall of said innermember, said piston means further comprising a piston rod secured tosaid piston and laterally spaced from said inner wall of said outermember and in slidable sealing relation to said outer and inner members,means for directing said fluid medium under pressure into the annularspace between said piston rod and inner wall of said outer member foraction upon said piston, said piston rod being laterally spaced fromsaid inner member to provide a second annular space therebetween, andmeans for directing said fluid medium under pressure into said secondannular space for action upon said piston. .Iaddend. .Iadd.
 25. On afloating drilling platform of the type having a derrick, an upper crownblock assembly having a plurality of sheaves mounted on the derrick, alower travelling block assembly having a plurality of sheaves, poweredcable means interconnecting the travelling block assembly and the crownblock assembly, a rotary carrier for supporting the drill string belowthe travelling block, and a hydraulic heave compensator between thetravelling block assembly and the rotary carrier; said crown block beingformed with a vertical opening passing between said sheaves and saidheave compensator comprising a central cylinder mounted on thetravelling block assembly and arranged to pass through said verticalopening while the travelling block is selectively raised by the cablemeans, a piston in said cylinder having a rod projecting downwardly tosaid carrier, and coupling means for selectively coupling said carrierto the travelling block assembly when the piston is raised in thecylinder. .Iaddend.